1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a backlight unit which enhances luminance and color reproduction quality of a LCD and a LCD device having the backlight unit.
2. Description of the Background Art
In general, a LCD device includes a thin film transistor (TFT) array substrate and a color filter substrate which face each other, a liquid crystal display panel having a liquid crystal layer formed between the two substrates, a driving unit for driving the liquid crystal panel, and a backlight unit for supplying light to the liquid crystal panel.
The TFT array substrate includes a plurality of data lines vertically arranged with an interval therebetween, a plurality of gate lines horizontally arranged with an interval therebetween, and a pixel defined at each crossing of the data line and the gate line such that the pixels are formed in a matrix form.
The color filter substrate includes a red, green and blue color filter layer formed at positions corresponding to the pixels. Additionally, the color filter also has a black matrix for preventing light leakage between the color filters. The black matrix prevents color interference of light passing through the pixels.
A common electrode and a pixel electrode face inner surfaces of the color filter substrate and the TFT array substrate, respectively. The common electrode and the pixel electrode apply an electric field to the liquid crystal layer. Here, the pixel electrode is formed on the TFT array substrate according to each pixel and the common electrode is integrally formed on the surface of the color filter substrate. An arrangement of the liquid crystal molecules in the liquid crystal layer may be controlled by controlling a voltage applied to the pixel electrode. Accordingly, light transmittance of the pixels may be individually controlled by controlling the voltage applied to the pixel electrode.
The backlight unit supplies light to the LCD device. When light emitted from the backlight unit passes through the liquid crystal layer, the arrangement of the liquid crystals controls light transmittance and an image is displayed. Typically, there are two types of backlight units, an edge type backlight unit and a direct type backlight unit. The type of unit depends on the position of lamps which are used as the light source.
In an edge type backlight unit, a lamp is disposed at either one side or both sides of the liquid crystal panel. Light generated from the lamp is projected on the screen of the liquid crystal panel with a light guide plate.
Direct type backlight units include a plurality of fluorescent lamps that are arranged in a row and irradiate light onto a surface of the liquid crystal panel. As the size of liquid crystal panels have increased to over 20 inches, greater emphasis has been placed on the development direct type backlight units. The direct type backlight unit is more efficient than the edge type backlight unit. Thus, the direct type backlight unit is commonly used for applications requiring high luminance, such as a large-screen LCD device.
Typically, a CCFL (Cold Cathode Fluorescent Lamp), an HCFL (Hot Cathode Fluorescent Lamp), an EL (Electro Luminescence), an LED (Light Emitting Diode), and the like, are used as light sources in related art backlight units. In addition, of the light sources, the CCFL and the LED lamp are most commonly used.
However, while a fluorescent lamp has luminance which is generally high and uniform, the color reproduction of a fluorescent lamp is subpar. In addition, while a LED or an EL lamp can control partial luminance and implement high quality color reproduction, the luminance and uniformity of LED and EL lamps are low when compared with the CCFL.